The present invention relates to a condenser microphone and more particularly, a condenser microphone provided with an impedance converter circuit of the push-pull type.
Various attempts have been tried to reduce the distortion of a condenser microphone and to make large the allowable input thereto. One of them is that an electrostatic transducer which obtains an electrical output signal responsive to an acoustic input signal or an impedance converter circuit for reducing the electric output impedance of this electrostatic transducer using two FETs (field effect transistor) is arranged in push-pull type.
The arrangement of the impedance converter circuit as a push-pull type is an effective way to enable a relatively simple circuit arrangement to reduce the harmonic distortion. The push-pull arrangement of impedance converter circuit is described in detail on pages 530-535, Vol. 23, J.A.E.S., for example. The impedance converter circuit described by this material comprises a complementary push-pull source follower consisting of an N-channel FET and a P-channel FET.
According to this impedance converter circuit, its output voltage varies from 0 V only to its power supply voltage. When the distortion factor is taken into consideration as a practical problem, it will follow that the allowable input level of this impedance circuit becomes substantially lower than its power supply voltage. According to the present inventor's tests, the allowable input level had a limit, 1 V in peak to peak and -9dB V (0dB V=1 V) in decibel notation, when its power supply voltage was 1.5 V. The allowable acoustic input level of the microphone naturally depends upon this value and often becomes unpractical when the allowable input level of impedance converter circuit takes such value.
It is considered at first that the power supply voltage is raised to increase the allowable input level of impedance converter circuit, so that the allowable acoustic input level may be raised. When dry cells are employed as a power supply, the number of cells may be increased or a DC-DC converter may be employed. However, the increase of cell number will cause the microphone to be larged-sized, which is not preferable in the case of portable microphone. If the DC-DC converter is employed, the power dissipation of cells will be remarkably hastened due to the power loss caused by the DC-DC converter. In addition, when an external power supply is employed instead of cells, it makes the handling of microphone troublesome.